Persistent Seismicity in Eastern-Central Italy: Evidence for a Complex Structural Asperity Dominating Mid-Crustal Deformation

Fault systems are inherently heterogeneous, with barriers and asperities exerting a first-order control on rupture propagation and on the spatio-temporal distribution of seismicity. The persistence of long-lived seismicity raises questions about whether earthquake activity is primarily governed by local structural complexities or by large-scale tectonic loading acting on simplified, homogeneous fault surfaces.

In this study, we report persistent mid-crustal seismicity in Eastern-Central Italy along the Adriatic Basal Thrust (ABT), a major compressional structure deepening westward from the Adriatic offshore to the Apennine Foothills. Its 3D geometric and kinematic architecture was reconstructed combining geological information and a high-resolution seismological dataset of relocated earthquakes and focal mechanisms (de Nardis et al., 2022). Specifically, the seismic catalogue was refined using recordings from the ReSIICO seismic network and a 3D velocity model (Cattaneo et al., 2019). The ABT extends ~210 km along strike and dips at ~20°, with its main internal splay corresponding to the Near Coast Thrust (NCT). Seismicity is unevenly distributed; while the northern sector hosts instrumental earthquakes mainly at upper crustal levels, the southern sector appears locked at the surface but accounts for ~75% of the total deep seismicity, dominated by low-magnitude events (M_L mode ~0.8–0.9).

To extend the temporal perspective, we analyzed the Italian seismic catalogue over 40 years (1985–2024) (https://terremoti.ingv.it/). Fractal analysis and space–time clustering identify three persistent seismicity clusters: two shallow clusters likely related to anthropogenic processes (i.e., quarry blasts) and a third, dominant cluster consistently associated with the ABT. The spatio-temporal analysis reveals that within this tectonic cluster, ~76% of seismicity consists of non-triggered events representing background tectonic loading, with only a few moderate episodes of spatio-temporal clustering.

The remarkable long-term persistence of this activity prompted a deeper investigation into the underlying fault architecture through the high-resolution seismic catalogue. This analysis revealed that the seismicity highlights a complex structural duplex acting as a geometric asperity in the linkage zone between the ABT and the internal splay. This mid-crustal segment consists of two low-angle, west-dipping splays interconnected by high-angle ramps, forming a structural knot that hinders smooth slip. Overall, the spatial persistence, depth distribution, and geometric complexity of the microseismicity indicate that fault-scale heterogeneities and structural jams dominate over large-scale regional coupling. This implies that the continuous release of seismic energy within these complex structural nodes acts as a mechanical accommodation process, effectively controlling the segmentation and the maximum rupture potential of the entire fault system.

Cattaneo, M., Frapiccini, M., Ladina, C., Marzorati, S. & Monachesi, G. A mixed automatic-manual seismic catalog for Central-Eastern Italy: Analysis of homogeneity. Ann. Geophys. (2017).

de Nardis, R., Pandolfi, C., Cattaneo, M. et al. Lithospheric double shear zone unveiled by microseismicity in a region of slow deformation. Sci Rep 12, 21066 (2022).